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The Surface Water and Ocean Topography (SWOT) altimeter mission can measure high-resolution wide-swath sea surface heights (SSHs) that may greatly improve the current accuracy and spatial resolution of marine gravity from nadir-looking altimeters. To investigate the potential of SWOT in recovering high-quality marine gravity and how SWOT observation errors should be treated to optimize the accuracy of gravity anomaly from SWOT observations, we create high-wavenumber SSH components from multi-beam depths in the northern South China Sea (SCS) and simulate SWOT SSH errors. To cross-validate gravity signals and avoid gravity errors from SWOT, we use two computational methods (inverse Vening-Meinesz formula, IVM and inverse Stokes' integral, ISM) and recommend separate optimal data processing strategies when using geoid gradients (GGs) and geoid heights (GHs) for gravity recovery. The use of GGs (for IVM) effectively eliminates systematic errors in gravity derivation. If GHs (for ISM) are used in gravity recovery, the tilt in wide-swath SSHs should be removed before gravity computation, and the recovered gravity must be filtered (post-processed) to avoid artificial gravity signals due to the SSH errors. Our assessments using mgal-accuracy shipborne gravity anomalies in the northern SCS show that multiple-cycle SWOT observations can deliver high-quality marine gravity anomalies. IVM is more robust than ISM in resisting random and systematic errors in SWOT. Our processing strategies can be used for the gravity validation of SSHs from SWOT's fast-sampling and science phases.
Daocheng Yu; Cheinway Hwang; Ole Baltazar Andersen; Emmy T.Y. Chang; Lucile Gaultier. Gravity recovery from SWOT altimetry using geoid height and geoid gradient. Remote Sensing of Environment 2021, 265, 112650 .
AMA StyleDaocheng Yu, Cheinway Hwang, Ole Baltazar Andersen, Emmy T.Y. Chang, Lucile Gaultier. Gravity recovery from SWOT altimetry using geoid height and geoid gradient. Remote Sensing of Environment. 2021; 265 ():112650.
Chicago/Turabian StyleDaocheng Yu; Cheinway Hwang; Ole Baltazar Andersen; Emmy T.Y. Chang; Lucile Gaultier. 2021. "Gravity recovery from SWOT altimetry using geoid height and geoid gradient." Remote Sensing of Environment 265, no. : 112650.
Land subsidence is a significant problem around the world that can increase the risk of flooding, damage to infrastructure, and economic loss. Hence, the continual monitoring of subsidence is important for early detection, mechanism understanding, countermeasure implementation, and deformation prediction. In this study, we used multiple-sensor observations from the Continuous Global Positioning System (CGPS), the small baseline subset (SBAS) algorithm, interferometric synthetic-aperture radar (InSAR), precise leveling, multi-layer compaction monitoring wells (MLCWs), and groundwater observation wells (GWs) to show the spatial and temporal details of land subsidence in the Choushui River alluvial fan (CRAF), Taiwan, from 1993 to 2019. The results showed that significant land subsidence has occurred along the coastal areas in the CRAF, and most of the inland subsidence areas have also experienced higher subsidence rates (>30 mm/yr). The analysis of subsidence along the Taiwan High Speed Rail (THSR) revealed a newly formed subsidence center between Tuku and Yuanchang Townships in Yunlin, with high subsidence rates ranging from 30 to 70 mm/yr. We propose a map showing, for the first time, the distribution of deep compactions occurring below 300 m depth in the CRAF.
Yi-An Chen; Chung-Pai Chang; Wei-Chia Hung; Jiun-Yee Yen; Chih-Heng Lu; Cheinway Hwang. Space-Time Evolutions of Land Subsidence in the Choushui River Alluvial Fan (Taiwan) from Multiple-Sensor Observations. Remote Sensing 2021, 13, 2281 .
AMA StyleYi-An Chen, Chung-Pai Chang, Wei-Chia Hung, Jiun-Yee Yen, Chih-Heng Lu, Cheinway Hwang. Space-Time Evolutions of Land Subsidence in the Choushui River Alluvial Fan (Taiwan) from Multiple-Sensor Observations. Remote Sensing. 2021; 13 (12):2281.
Chicago/Turabian StyleYi-An Chen; Chung-Pai Chang; Wei-Chia Hung; Jiun-Yee Yen; Chih-Heng Lu; Cheinway Hwang. 2021. "Space-Time Evolutions of Land Subsidence in the Choushui River Alluvial Fan (Taiwan) from Multiple-Sensor Observations." Remote Sensing 13, no. 12: 2281.
The comprehensive information of global terrestrial water storage (TWS) components (soil moisture, groundwater, snow, surface water) is essential for effective assessment of water resource availability, climate variation, and disaster mitigation measures. Observational data provided by the Gravity Recovery And Climate Experiment (GRACE) and GRACE Follow-On satellite missions offer global TWS variation (ΔTWS) in terms of an integrated water column. However, GRACE spatial resolution is relatively coarse (i.e., 3°), and the vertically integrated value cannot be separated into ΔTWS components directly. This study demonstrates the feasibility to estimate ΔTWS components at any desired spatial-vertical resolution by effectively maintaining the native resolution of the employed hydrological knowledge. It utilizes a least-squares with constraints (LSC) approach to rigorously incorporate GRACE and GRACE-FO data and a priori hydrological knowledge, with the aim to improve global ΔTWS components’ accuracy and spatial resolution. The 3°×3° GRACE mascon derived ΔTWS data is disaggregated into the 0.5°×0.5° anomalous soil moisture storage (ΔSMS), groundwater storage (ΔGWS), snow water equivalent (ΔSWE), and surface water storage (ΔSWS) based on the covariance information obtained from the Community Atmosphere Biosphere Land Exchange (CABLE) and the PCRaster Global Water Balance (PCR-GLOBWB) models. Evaluation with different ground measurements and satellite products between 2002 and 2019 exhibits significantly improved accuracy in all individual ΔTWS components. This improvement is of particular note in ΔGWS and ΔSWS, where the LSC approach increases the globally averaged correlation values by approximately 0.13 and 0.05, respectively. Reliable prior knowledge leads to a more accurate ΔTWS component estimate, and the use of ensemble-mean knowledge yields the best result.
Natthachet Tangdamrongsub; Cheinway Hwang; Jordan S. Borak; Saowanit Prabnakorn; Jiancheng Han. Optimizing GRACE/GRACE-FO data and a priori hydrological knowledge for improved global terrestial water storage component estimates. Journal of Hydrology 2021, 598, 126463 .
AMA StyleNatthachet Tangdamrongsub, Cheinway Hwang, Jordan S. Borak, Saowanit Prabnakorn, Jiancheng Han. Optimizing GRACE/GRACE-FO data and a priori hydrological knowledge for improved global terrestial water storage component estimates. Journal of Hydrology. 2021; 598 ():126463.
Chicago/Turabian StyleNatthachet Tangdamrongsub; Cheinway Hwang; Jordan S. Borak; Saowanit Prabnakorn; Jiancheng Han. 2021. "Optimizing GRACE/GRACE-FO data and a priori hydrological knowledge for improved global terrestial water storage component estimates." Journal of Hydrology 598, no. : 126463.
Ever decreasing water resources and climate change have driven the increasing use of groundwater causing land subsidence in many countries. Geodetic sensors such as InSAR, GPS and leveling can detect surface deformation, but cannot measure subsurface deformation. A single‐well, single‐depth extensometer can be used to measure subsurface deformation, but it cannot delineate the depths of major compaction and provide insight about the deformation mechanism throughout a complex aquifer system, unless many extensometers at different depths are used. We present a multi‐layer compaction well (MLCW), installed in a borehole, that uses magnetic rings to detect stratum compaction at 25 depths as deep as 300 m below land surface. Our laboratory and field assessments indicate one‐mm precision and accuracy for one single‐depth magnetic reading. We tested the performance of MLCW by measuring aquifer‐system compaction over the proximal, middle and distal fans of the Choushui River Alluvial Fan (CRAF) that has long experienced severe land subsidence. The MLCW measurements were used to create time‐depth diagrams of compaction showing different compaction rates at different layers of aquifers and aquitards to identify the depths of major compactions. The elastic (reversible) and inelastic (irreversible) compactions from MLCW were used in stress‐strain analyses to estimate skeletal specific storages and the safe groundwater levels, below which groundwater extractions have caused irreversible compactions. The hydrogeological parameters derived from MLCW measurements can help governmental agencies to determine effective land‐use and water‐use policies, and ascertain the best strategy for utilizing artificial recharge to prevent land subsidence and achieve sustainable groundwater management.
Wei‐Chia Hung; Cheinway Hwang; Michelle Sneed; Yi‐An Chen; Chi‐Hua Chu; Shao‐Hung Lin. Measuring and Interpreting Multilayer Aquifer‐System Compactions for a Sustainable Groundwater‐System Development. Water Resources Research 2021, 57, 1 .
AMA StyleWei‐Chia Hung, Cheinway Hwang, Michelle Sneed, Yi‐An Chen, Chi‐Hua Chu, Shao‐Hung Lin. Measuring and Interpreting Multilayer Aquifer‐System Compactions for a Sustainable Groundwater‐System Development. Water Resources Research. 2021; 57 (4):1.
Chicago/Turabian StyleWei‐Chia Hung; Cheinway Hwang; Michelle Sneed; Yi‐An Chen; Chi‐Hua Chu; Shao‐Hung Lin. 2021. "Measuring and Interpreting Multilayer Aquifer‐System Compactions for a Sustainable Groundwater‐System Development." Water Resources Research 57, no. 4: 1.
Accurate and densely covered specific yields (S y ) are essential for estimating the storage capacity of a groundwater reservoir. A cross‐well pumping test can determine S y , but its high cost often makes it unsuitable for sampling high‐resolution S y . The gravity‐based method (GBM) based on gravity changes near existing groundwater wells may outperform cross‐well pumping tests in estimating S y . We established ten gravity sites close to groundwater wells in the aquifer‐rich Choushui River Alluvial Fan and Mingchu Basin in central Taiwan and measured gravity changes with two FG5 gravimeters from 2012 to 2017. Thirty‐nine S y values with formal errors are determined by natural rises and falls in gravity and groundwater level. The representative S y values (0.04 to 0.29) from GBM are in general consistent with those from cross‐well pumping tests (0.03 to 0.24). Repeated groundwater level changes over similar depth range at different times serve as revisit tests, showing that GBM can reproduce a reliable S y value at a given site and depth. Soil moisture and compaction data show that the effects of gravity changes originating from unsaturated zones and deep aquifer layers are minor. Using the cylinder model for aquifers with limited lateral extents, we assess the validity of the Bouguer model by quantifying gravity differences and relative S y differences originating from the model assumption. Improvements in environmental resilience and transportability achieved by recent atomic gravimeters may increase the potential of GBM to replace or supplement cross‐well pumping tests in densifying S y point densities for an improved groundwater resource management.
Kuan‐Hung Chen; Cheinway Hwang; Liang‐Cheng Chang; Jui‐Pin Tsai; Tian‐Chyi Jim Yeh; Ching‐Chung Cheng; Chien‐Chung Ke; Wei Feng. Measuring Aquifer Specific Yields With Absolute Gravimetry: Result in the Choushui River Alluvial Fan and Mingchu Basin, Central Taiwan. Water Resources Research 2020, 56, 1 .
AMA StyleKuan‐Hung Chen, Cheinway Hwang, Liang‐Cheng Chang, Jui‐Pin Tsai, Tian‐Chyi Jim Yeh, Ching‐Chung Cheng, Chien‐Chung Ke, Wei Feng. Measuring Aquifer Specific Yields With Absolute Gravimetry: Result in the Choushui River Alluvial Fan and Mingchu Basin, Central Taiwan. Water Resources Research. 2020; 56 (9):1.
Chicago/Turabian StyleKuan‐Hung Chen; Cheinway Hwang; Liang‐Cheng Chang; Jui‐Pin Tsai; Tian‐Chyi Jim Yeh; Ching‐Chung Cheng; Chien‐Chung Ke; Wei Feng. 2020. "Measuring Aquifer Specific Yields With Absolute Gravimetry: Result in the Choushui River Alluvial Fan and Mingchu Basin, Central Taiwan." Water Resources Research 56, no. 9: 1.
This paper combines gravity data collected from airborne, shipborne and terrestrial surveys and those derived from satellite altimetry to determine a high-resolution gravimetric and hybrid geoid model (on a 30” × 30″ grid) in and around Taiwan. Some 6000 new land gravity values at a 0.03-mGal precision make a notable contribution to the geoid modeling. Shipborne gravity data in waters 20 km offshore Taiwan were collected to improve the coastal geoid precision. In a circular area of 50 km around each of the five major tide gauges in Taiwan, gravity data were measured to improve vertical datum connections between Taiwan and its four offshore islands. Height anomalies were computed first and then converted to geoid heights. At > 2000 benchmarks, we obtained measured geoid heights to assess the gravimetric-only geoid and to create a hybrid geoid. Our assessments and formal errors from least-squares collocation indicate few cm of standard deviations for both geoid models, but the gravimetric geoid has mean differences of up to 20 cm with the measured geoidal heights. The hybrid geoid is used in RTK-VBS orthometric heighting, achieving a 5-cm precision. The gravimetric geoid is used to determine the relative differences in the ocean’s mean dynamic topography (MDT) between Taiwan and the four offshore islands, which are also compared with those from oceanic and altimetric methods for estimating MDT. Differences in MDT help to identify 41.7 cm and 54.1 cm offsets in the current vertical datums of Penghu and Lanyu islands. In a low-lying, flood-prone region of southern Taiwan, the hybrid geoid improves LiDAR mapping of sub-zero elevation zones by 20 cm, corresponding to 70 years of sea level rise at an assumed rate of 0.286 cm/yr.
Cheinway Hwang; Hung-Jui Hsu; W. E. Featherstone; Ching-Chung Cheng; Ming Yang; Wenhsuan Huang; Chong-You Wang; Jiu-Fu Huang; Kwo-Hwa Chen; Chi-Hsun Huang; Hechin Chen; Wen-Yi Su. New gravimetric-only and hybrid geoid models of Taiwan for height modernisation, cross-island datum connection and airborne LiDAR mapping. Bulletin Géodésique 2020, 94, 1 -22.
AMA StyleCheinway Hwang, Hung-Jui Hsu, W. E. Featherstone, Ching-Chung Cheng, Ming Yang, Wenhsuan Huang, Chong-You Wang, Jiu-Fu Huang, Kwo-Hwa Chen, Chi-Hsun Huang, Hechin Chen, Wen-Yi Su. New gravimetric-only and hybrid geoid models of Taiwan for height modernisation, cross-island datum connection and airborne LiDAR mapping. Bulletin Géodésique. 2020; 94 (9):1-22.
Chicago/Turabian StyleCheinway Hwang; Hung-Jui Hsu; W. E. Featherstone; Ching-Chung Cheng; Ming Yang; Wenhsuan Huang; Chong-You Wang; Jiu-Fu Huang; Kwo-Hwa Chen; Chi-Hsun Huang; Hechin Chen; Wen-Yi Su. 2020. "New gravimetric-only and hybrid geoid models of Taiwan for height modernisation, cross-island datum connection and airborne LiDAR mapping." Bulletin Géodésique 94, no. 9: 1-22.
Extracting groundwater for agricultural, aquacultural, and industrial use in central Taiwan has caused large-scale land subsidence that poses a threat to the operation of the Taiwan High Speed Railway near Yunlin County. We detected Yunlin subsidence using the Sentinel-1A Synthetic Aperture Radar (SAR) by the Small BAseline Subset (SBAS) method from April 2016 to April 2017. We calibrated the initial InSAR-derived displacement rates using GPS measurements and reduced the velocity difference between the two sensors from 15.0 to 8.5 mm/a. In Yunlin’s severe subsidence regions, cumulative displacements from InSAR and GPS showed that the dry-season subsidence contributed 60%–74% of the annual subsidence. The InSAR-derived vertical velocities matched the velocities from leveling to better than 10 mm/a. In regions with few leveling measurements, InSAR increased the spatial resolution of the vertical velocity field and identified two previously unknown subsidence spots over an industrial zone and steel factory. Annual significant subsidence areas (subsidence rate > 30 mm/a) from leveling from 2011 to 2017 increased with the declining dry-season rainfalls, suggesting that the dry-season rainfall was the deciding factor for land subsidence. A severe drought in 2015 (an El Niño year) dramatically increased the significant subsidence area to 659 km2. Both InSAR and leveling detected similarly significant subsidence areas in 2017, showing that InSAR was an effective technique for assessing whether a subsidence mitigation measure worked. The newly opened Hushan Reservoir can supply surface water during dry seasons and droughts to counter rain shortage and can thereby potentially reduce land subsidence caused by groundwater extraction.
Yi-Jie Yang; Cheinway Hwang; Wei-Chia Hung; Thomas Fuhrmann; Yi-An Chen; Shiang-Hung Wei. Surface Deformation from Sentinel-1A InSAR: Relation to Seasonal Groundwater Extraction and Rainfall in Central Taiwan. Remote Sensing 2019, 11, 2817 .
AMA StyleYi-Jie Yang, Cheinway Hwang, Wei-Chia Hung, Thomas Fuhrmann, Yi-An Chen, Shiang-Hung Wei. Surface Deformation from Sentinel-1A InSAR: Relation to Seasonal Groundwater Extraction and Rainfall in Central Taiwan. Remote Sensing. 2019; 11 (23):2817.
Chicago/Turabian StyleYi-Jie Yang; Cheinway Hwang; Wei-Chia Hung; Thomas Fuhrmann; Yi-An Chen; Shiang-Hung Wei. 2019. "Surface Deformation from Sentinel-1A InSAR: Relation to Seasonal Groundwater Extraction and Rainfall in Central Taiwan." Remote Sensing 11, no. 23: 2817.
Precise measurements of height changes (HCs) are important for improved estimates of mass balance of the Greenland Ice Sheet (GrIS). Here we determine 10 years of precise, high‐resolution HCs of the GrIS from Envisat radar altimeter using a subwaveform retracker and a modified repeat‐track method. The HCs show clear seasonal changes and monotonic declines over glaciers on the coasts such as Zachariae Isstrøm. We enhance mass‐change estimates from GRACE data using HCs and densities from interannual correlations between GRACE‐derived mass changes and HCs. We estimate the mass changes of eight drainage basins with our combined mass change. The largest mass change between 2002 and 2012 occurred in the northwest basin and the smallest in the northeast basin. We separate the ice and snow HC rates to derive a ratio (f) between them to characterize the relative importance of ice or snow to mass change. The snow HC rates are mostly positive over the GrIS, except on the margins of the west coast and Zachariae Isstrøm. The mean ice HC rate is ‐6.6±3.9 cm yr‐1 over 2002–2006, which accelerated to ‐13.9±2.4 cm yr‐1 over 2007–2012. The f factors show a clear post‐2006 ice dominance in the GrIS mass loss, particularly on the west coast, with a mean 91.4% ice contribution over 2002–2006, increasing to 94.5% over 2007–2012. This indicates increasing mass loss after 2006. A coincident radar altimeter and gravimetry mission is important for studying mass balance and separating snow and ice contributions over space and time.
Yuande Yang; Fei Li; Cheinway Hwang; Minghu Ding; Jiangjun Ran. Space‐Time Evolution of Greenland Ice Sheet Elevation and Mass From Envisat and GRACE Data. Journal of Geophysical Research: Earth Surface 2019, 124, 2079 -2100.
AMA StyleYuande Yang, Fei Li, Cheinway Hwang, Minghu Ding, Jiangjun Ran. Space‐Time Evolution of Greenland Ice Sheet Elevation and Mass From Envisat and GRACE Data. Journal of Geophysical Research: Earth Surface. 2019; 124 (8):2079-2100.
Chicago/Turabian StyleYuande Yang; Fei Li; Cheinway Hwang; Minghu Ding; Jiangjun Ran. 2019. "Space‐Time Evolution of Greenland Ice Sheet Elevation and Mass From Envisat and GRACE Data." Journal of Geophysical Research: Earth Surface 124, no. 8: 2079-2100.
SUMMARY HY-2A is China's first satellite altimeter mission, launched in Aug. 2011. Its geodetic mission (GM) started from 2016 March 30 till present, collecting sea surface heights for about five 168-d cycles. To test how the HY-2A altimeter performs in marine gravity derivation, we use the least-squares collocation method to determine marine gravity anomalies on 1′ × 1′ grids around the South China Sea (covering 0°–30°N, 105°E–125°E) from the HY-2A/GM-measured geoid gradients. We assess the qualities of the HY-2A/GM-derived gravity over different depths and areas using the bias and tilt-adjusted ship-borne gravity anomalies from the U.S. National Centers for Environmental Information (NCEI) and the Second Institute of Oceanography, Ministry of Natural Resources (MNR) of P. R. China. The RMS difference between the HY-2A/GM-derived and the NCEI ship-borne gravity is 5.91 mGal, and is 5.33 mGal when replacing the HY-2A value from the Scripps Institution of Oceanography (SIO) V23.1 value. The RMS difference between the HY-2A/GM-derived and the MNR ship-borne gravity is 2.90 mGal, and is 2.76 mGal when replacing the HY-2A value from the SIO V23.1 value. The RMS difference between the HY-2A and SIO V23.1 value is 3.57 mGal in open sea areas at least 20 km far away from the coast. In general, the difference between the HY-2A/GM-derived gravity and ship-borne gravity decreases with decreasing gravity field roughness and increasing depth. HY-2A results in the lowest gravity accuracy in areas with islands or reefs. Our assessment result suggests that HY-2A can compete with other Ku-band altimeter missions in marine gravity derivation.
Chengcheng Zhu; Jinyun Guo; Cheinway Hwang; Jinyao Gao; Jiajia Yuan; Xin Liu. How HY-2A/GM altimeter performs in marine gravity derivation: assessment in the South China Sea. Geophysical Journal International 2019, 219, 1056 -1064.
AMA StyleChengcheng Zhu, Jinyun Guo, Cheinway Hwang, Jinyao Gao, Jiajia Yuan, Xin Liu. How HY-2A/GM altimeter performs in marine gravity derivation: assessment in the South China Sea. Geophysical Journal International. 2019; 219 (2):1056-1064.
Chicago/Turabian StyleChengcheng Zhu; Jinyun Guo; Cheinway Hwang; Jinyao Gao; Jiajia Yuan; Xin Liu. 2019. "How HY-2A/GM altimeter performs in marine gravity derivation: assessment in the South China Sea." Geophysical Journal International 219, no. 2: 1056-1064.
S Gautier; C Tiberi; M Lopez; O Foix; S Lallemand; T Theunissen; C Hwang; E Chang. Detailed lithospheric structure of an arc-continent collision beneath Taiwan revealed by joint inversion of seismological and gravity data. Geophysical Journal International 2019, 1 .
AMA StyleS Gautier, C Tiberi, M Lopez, O Foix, S Lallemand, T Theunissen, C Hwang, E Chang. Detailed lithospheric structure of an arc-continent collision beneath Taiwan revealed by joint inversion of seismological and gravity data. Geophysical Journal International. 2019; ():1.
Chicago/Turabian StyleS Gautier; C Tiberi; M Lopez; O Foix; S Lallemand; T Theunissen; C Hwang; E Chang. 2019. "Detailed lithospheric structure of an arc-continent collision beneath Taiwan revealed by joint inversion of seismological and gravity data." Geophysical Journal International , no. : 1.
Satellite altimetry has been successfully applied to monitoring water level variation of global lakes. However, it is still difficult to retrieve accurate and continuous observations for most Tibetan lakes, due to their high altitude and rough terrain. Aiming to generate long-term and accurate lake level time series for the Tibetan lakes using multi-altimeters, we present a robust strategy including atmosphere delay corrections, waveform retracking, outlier removal and inter-satellite bias adjustment. Apparent biases in dry troposphere corrections from different altimeter products are found, and such correctios must be recalculated using the same surface pressure model. A parameter is defined to evaluate the performance of the retracking algorithm. The ICE retracker outperforms the 20% and 50% threshold retrackers in the case of Ngangzi Co, where a new wetland has been established. A two-step algorithm is proposed for outlier removal. Two methods are adopted to estimate inter-satellite bias for different cases of with and without overlap. Finally, a 25-year-long lake level time series of Ngangzi Co are constructed using the TOPEX/Poseidon-family altimeter data from October 1992 to December 2017, resulting in an accuracy of ~17 cm for TOPEX/Poseidon and ~10 cm for Jason-1/2/3. The accuracy of retrieved lake levels is on the order of decimeter. Because of no gauge data available, ICESat and SARAL data with the accuracy better than 7 cm are used for validation. A correlation more than 0.9 can be observed between the mean lake levels from TOPEX/Poseidon-family satellites, ICESat and SARAL. Compared to the previous studies and other available altimeter-derived lake level databases, our result is the most robust and has resulted in the maximum number of continuous samples. The time series indicates that the lake level of Ngangzi Co increased by ~8 m over 1998–2017 and changed with different rates in the past 25 years (-0.39 m/yr in 1992–1997, 1.03 m/yr in 1998–2002 and 0.32 m/yr in 2003–2014). These findings will enhance the understanding of water budget and the effect of climate change.
Cheinway Hwang; Yonghai Chu; Zhengkai Huang; Nengfang Chao. Robust, Long-term Lake Level Change from Multiple Satellite Altimeters in Tibet: Observing the Rapid Rise of Ngangzi Co over a New Wetland. Remote Sensing 2019, 11, 558 .
AMA StyleCheinway Hwang, Yonghai Chu, Zhengkai Huang, Nengfang Chao. Robust, Long-term Lake Level Change from Multiple Satellite Altimeters in Tibet: Observing the Rapid Rise of Ngangzi Co over a New Wetland. Remote Sensing. 2019; 11 (5):558.
Chicago/Turabian StyleCheinway Hwang; Yonghai Chu; Zhengkai Huang; Nengfang Chao. 2019. "Robust, Long-term Lake Level Change from Multiple Satellite Altimeters in Tibet: Observing the Rapid Rise of Ngangzi Co over a New Wetland." Remote Sensing 11, no. 5: 558.
Specific yield (Sy) and specific storage (Ss) are two hydrogeological parameters often estimated in a costly pumping test (hydraulic method). Pumping or a natural aquifer recession can cause gravity change and surface deformation, useful for Sy and Ss determination by the more economic geodetic method. From 2013 to 2017, over few days we measured gravity changes at 4 sites and subsidence at 1 site in Taiwan by a FG5 gravimeter and a precision level to estimate Sy and Ss. Using short‐time gravity and level records avoids complicated logistic supports and temporal effects. The measured gravity changes are associated with the MODFLOW‐modeled groundwater depletion. We succeeded in estimating Sy at two sites and failed at the other two. One successful case uses rapid post‐rain declines of gravity and groundwater level. We did leveling only at one site, but here the 500‐minutes, continuous, sub‐mm subsidence records result in a Ss consistent with the hydraulic result. Lessons on the failed cases of Sy are suggested to avoid disrupting factors in gravity surveys. A semi‐automatic leveling procedure is proposed to maximize the chance for Ss determination. Our simulations show that gravity changes increase with Sy and decrease with initial hydraulic head and horizontal gravimeter‐pumping well distance, helping to predict if the gravity‐based method works at a site and to best plan a gravity survey. Drilling data show that the Sy values from the hydraulic and geodetic methods represent aquifer storage coefficients over different aquifer layers.
Kuan‐Hung Chen; Cheinway Hwang; Liang‐Cheng Chang; Chien‐Chung Ke. Short‐Time Geodetic Determination of Aquifer Storage Coefficient in Taiwan. Journal of Geophysical Research: Solid Earth 2018, 1 .
AMA StyleKuan‐Hung Chen, Cheinway Hwang, Liang‐Cheng Chang, Chien‐Chung Ke. Short‐Time Geodetic Determination of Aquifer Storage Coefficient in Taiwan. Journal of Geophysical Research: Solid Earth. 2018; ():1.
Chicago/Turabian StyleKuan‐Hung Chen; Cheinway Hwang; Liang‐Cheng Chang; Chien‐Chung Ke. 2018. "Short‐Time Geodetic Determination of Aquifer Storage Coefficient in Taiwan." Journal of Geophysical Research: Solid Earth , no. : 1.
The increase of anthropogenic activities has triggered global sea level rise to threaten many low-lying and unprotected coastal areas. Without measures, global sea levels will continue to rise at an accelerating rate in the 21st century. This paper quantifies sea level trends around the Malaysian seas using measurements from multiple altimeter missions over 1993-2015. Sea level anomalies (SLAs) are determined using data from the Radar Altimeter Database System (RADS) covering 8 altimeter missions. We use an enhanced processing strategy to optimize sea surface heights from RADS for the derivation of SLAs, including filtering, data gridding and moving average. Tidal height measurements at eight tide gauge stations around Peninsular Malaysia and East Malaysia are used to assess SLAs from altimetry. Our assessment results in similar patterns of SLAs, high correlation coefficients (>0.9) and small (few cm) root mean square differences (RMSE) between SLAs from altimetry and tide gauges over the same period. Sea level trends are determined by the robust fit regression analysis for the SLA time series. Our result shows that sea level rise trends around Malaysia range from 3.27 ± 0.12 mm yr-1 off eastern Malaysia to 4.95 ± 0.15 mm yr-1 west of Malaysia. Over 1993-2015, the mean rising rate around Malaysia is 4.22± 0.12 mm yr-1, and the cumulative sea level rise is 0.05m. This paper predicts the impact of such rising sea levels on environment, urban planning and climatology in the coastal areas of Malaysia.
Amalina Izzati Abdul Hamid; Ami Hassan Md Din; Cheinway Hwang; Nur Fadila Khalid; Astina Tugi; Kamaludin Mohd Omar. Contemporary sea level rise rates around Malaysia: Altimeter data optimization for assessing coastal impact. Journal of Asian Earth Sciences 2018, 166, 247 -259.
AMA StyleAmalina Izzati Abdul Hamid, Ami Hassan Md Din, Cheinway Hwang, Nur Fadila Khalid, Astina Tugi, Kamaludin Mohd Omar. Contemporary sea level rise rates around Malaysia: Altimeter data optimization for assessing coastal impact. Journal of Asian Earth Sciences. 2018; 166 ():247-259.
Chicago/Turabian StyleAmalina Izzati Abdul Hamid; Ami Hassan Md Din; Cheinway Hwang; Nur Fadila Khalid; Astina Tugi; Kamaludin Mohd Omar. 2018. "Contemporary sea level rise rates around Malaysia: Altimeter data optimization for assessing coastal impact." Journal of Asian Earth Sciences 166, no. : 247-259.
Unlike a typical remote sensing satellite that has a global coverage and non-integral orbital revolutions per day, Taiwan’s FORMOSAT-2 (FS-2) satellite has a non-global coverage due to the mission requirements of one-day repeat cycle and daily visit around Taiwan. These orbital characteristics result in an integer number of revolutions a day and orbital resonances caused by certain components of the Earth’s gravity field. Orbital flight data indicated amplified variations in the amplitudes of FS-2’s Keplerian elements. We use twelve years of orbital observations and maneuver data to analyze the cause of the resonances and explain the differences between the simulated (at the pre-launch stage) and real orbits of FS-2. The differences are quantified using orbital perturbation theories that describe secular and long-period orbital evolutions caused by resonances. The resonance-induced orbital rising rate of FS-2 reaches +1.425 m/day, due to the combined (modeled) effect of resonances and atmospheric drags (the relative modeling errors < 10%). The concave shapes in the time-evolution of the longitude of descending node (LonDN) coincide with the positive rates of daily semi-major axis (SMA) change, also caused by resonances. The non-zonal geopotential coefficients causing the resonance effects contributed up to 45% of FS-2’s inclination decline. Our retrospective analysis of FS-2’s resonant orbit can provide lessons for a remote sensing mission similar to FS-2, especially in the early mission design and planning phase.
Shin-Fa Lin; Cheinway Hwang. Orbital resonances of Taiwan’s FORMOSAT-2 remote sensing satellite. Acta Astronautica 2018, 147, 71 -85.
AMA StyleShin-Fa Lin, Cheinway Hwang. Orbital resonances of Taiwan’s FORMOSAT-2 remote sensing satellite. Acta Astronautica. 2018; 147 ():71-85.
Chicago/Turabian StyleShin-Fa Lin; Cheinway Hwang. 2018. "Orbital resonances of Taiwan’s FORMOSAT-2 remote sensing satellite." Acta Astronautica 147, no. : 71-85.
Chiayi County is located in the largest alluvial plain of Taiwan with extensive aquaculture and rice farming sustained by water extracted from groundwater wells. Chiayi is a typical aquaculture area affected by land subsidence, yet such lands worldwide combine to provide nearly 90% of global aquaculture products, greatly reducing oceanic overfishing problems. This study uses precision leveling, multi-layer compaction monitoring well (MLCW) and spaceborne SAR interferometry (InSAR) to examine the cause and effect of land subsidence in Chiayi associated with groundwater extractions and changes. Heights at benchmarks in a leveling network are measured annually and soil compactions at 24–26 layers up to 300-m depths at 7 MLCWs are collected at one-month intervals. Over 2007–2011, 15 ALOS/PALSAR images are processed by the method of TCPInSAR to produce subsidence rates. All sensors show that land subsidence occur in most parts of Chiayi, with rates reaching 4.5 cm/year around its coast, a result of groundwater pumping from shallow to deep aquifers. MLCWs detect mm-accuracy seasonal soil compactions coinciding with groundwater level fluctuations and causing dynamic compactions. Compactions near Taiwan High Speed Rail may reduce the strength of the rail’s supporting columns to degrade its safety. The SAR images yield subsidence rates consistent with those from leveling and compaction wells after corrections for systematic errors by the leveling result. Subsidence in Chiayi’s coastal area leads to relative sea level rises at rates up to 15 times larger than the global eustatic sea level rising rate, a risk typical for world’s aquaculture-rich regions. At the fish pond-covered Budai Township, InSAR identifies subsidence spots not detected by leveling, providing crucial geo-information for a sustainable land management for aquaculture industry.
Wei-Chia Hung; Cheinway Hwang; Yi-An Chen; Lei Zhang; Kuan-Hung Chen; Shiang-Hung Wei; Da-Ren Huang; Shu-Han Lin. Land Subsidence in Chiayi, Taiwan, from Compaction Well, Leveling and ALOS/PALSAR: Aquaculture-Induced Relative Sea Level Rise. Remote Sensing 2017, 10, 40 .
AMA StyleWei-Chia Hung, Cheinway Hwang, Yi-An Chen, Lei Zhang, Kuan-Hung Chen, Shiang-Hung Wei, Da-Ren Huang, Shu-Han Lin. Land Subsidence in Chiayi, Taiwan, from Compaction Well, Leveling and ALOS/PALSAR: Aquaculture-Induced Relative Sea Level Rise. Remote Sensing. 2017; 10 (2):40.
Chicago/Turabian StyleWei-Chia Hung; Cheinway Hwang; Yi-An Chen; Lei Zhang; Kuan-Hung Chen; Shiang-Hung Wei; Da-Ren Huang; Shu-Han Lin. 2017. "Land Subsidence in Chiayi, Taiwan, from Compaction Well, Leveling and ALOS/PALSAR: Aquaculture-Induced Relative Sea Level Rise." Remote Sensing 10, no. 2: 40.
It is widely believed that altimetry-derived sea surface heights (SSHs) in coastal zones are seriously degraded due to land contamination in altimeter waveforms from non-marine surfaces or due to inhomogeneous sea state conditions. Spurious peaks superimposed in radar waveforms adversely impact waveform retracking and hence require tailored algorithms to mitigate this problem. Here, we present an improved method to decontaminate coastal waveforms based on the waveform modification concept. SSHs within 10 km offshore are calculated from Jason-2 data by a 20% threshold retracker using decontaminated waveforms (DW-TR) and compared with those using original waveforms and modified waveforms in four study regions. We then compare our results with retracked SSHs in the sensor geophysical data record (SGDR) and with the state-of-the-art PISTACH (Prototype Innovant de Système de Traitement pour les Applications Côtières et l’Hydrologie) and ALES (Adaptive Leading Edge Subwaveform) products. Our result indicates that the DW-TR is the most robust retracker in the 0–10 km coastal band and provides consistent accuracy up to 1 km away from the coastline. In the four test regions, the DW-TR retracker outperforms other retrackers, with the smallest averaged standard deviations at 15 cm and 20 cm, as compared against the EGM08 (Earth Gravitational Model 2008) geoid model and tide gauge data, respectively. For the SGDR products, only the ICE retracker provides competitive SSHs for coastal applications. Subwaveform retrackers such as ICE3, RED3 and ALES perform well beyond 8 km offshore, but seriously degrade in the 0–8 km strip along the coast.
Zhengkai Huang; Haihong Wang; Zhicai Luo; C. K. Shum; Kuo-Hsin Tseng; Bo Zhong. Improving Jason-2 Sea Surface Heights within 10 km Offshore by Retracking Decontaminated Waveforms. Remote Sensing 2017, 9, 1077 .
AMA StyleZhengkai Huang, Haihong Wang, Zhicai Luo, C. K. Shum, Kuo-Hsin Tseng, Bo Zhong. Improving Jason-2 Sea Surface Heights within 10 km Offshore by Retracking Decontaminated Waveforms. Remote Sensing. 2017; 9 (10):1077.
Chicago/Turabian StyleZhengkai Huang; Haihong Wang; Zhicai Luo; C. K. Shum; Kuo-Hsin Tseng; Bo Zhong. 2017. "Improving Jason-2 Sea Surface Heights within 10 km Offshore by Retracking Decontaminated Waveforms." Remote Sensing 9, no. 10: 1077.
Hydraulic conductivity and specific yield ( ) are important aquifer parameters, pertinent to groundwater resources management and protection. These parameters are commonly estimated through a traditional cross‐well pumping test. Employing the traditional approach to obtain detailed spatial distributions of the parameters over a large area is generally formidable. For this reason, this study proposes a stochastic method that integrates hydraulic head and time‐lapse gravity based on hydraulic tomography (HT) to efficiently derive the spatial distribution of K and over a large area. This method is demonstrated using several synthetic experiments. Results of these experiments show that the K and fields estimated by joint inversion of the gravity and head data set from sequential injection tests in unconfined aquifers are superior to those from the HT based on head data alone. We attribute this advantage to the mass constraint imposed on HT by gravity measurements. Besides, we find that gravity measurement can detect the change of aquifer's groundwater storage at kilometer scale, as such they can extend HT's effectiveness over greater volumes of the aquifer. Furthermore, we find that the accuracy of the estimated fields is improved as the number of the gravity stations is increased. The gravity station's location, however, has minor effects on the estimates if its effective gravity integration radius covers the well field.
Jui-Pin Tsai; Tian-Chyi Jim Yeh; Ching-Chung Cheng; Yuanyuan Zha; Liang-Cheng Chang; Cheinway Hwang; Yu-Li Wang; Yonghong Hao. Fusion of Time-Lapse Gravity Survey and Hydraulic Tomography for Estimating Spatially Varying Hydraulic Conductivity and Specific Yield Fields. Water Resources Research 2017, 53, 8554 -8571.
AMA StyleJui-Pin Tsai, Tian-Chyi Jim Yeh, Ching-Chung Cheng, Yuanyuan Zha, Liang-Cheng Chang, Cheinway Hwang, Yu-Li Wang, Yonghong Hao. Fusion of Time-Lapse Gravity Survey and Hydraulic Tomography for Estimating Spatially Varying Hydraulic Conductivity and Specific Yield Fields. Water Resources Research. 2017; 53 (10):8554-8571.
Chicago/Turabian StyleJui-Pin Tsai; Tian-Chyi Jim Yeh; Ching-Chung Cheng; Yuanyuan Zha; Liang-Cheng Chang; Cheinway Hwang; Yu-Li Wang; Yonghong Hao. 2017. "Fusion of Time-Lapse Gravity Survey and Hydraulic Tomography for Estimating Spatially Varying Hydraulic Conductivity and Specific Yield Fields." Water Resources Research 53, no. 10: 8554-8571.
Urban faults in Shenzhen are potential threat to city security and sustainable development. To improve the knowledge of the Shenzhen fault zone, interpretation and inversion of gravity data were carried out. Bouguer gravity covering the whole Shenzhen City was calculated with a 1-km resolution. Wavelet multi-scale analysis (MSA) was applied to the Bouguer gravity data to obtain the multilayer residual anomalies corresponding to different depths. In addition, 2D gravity models were constructed along three profiles. The Bouguer gravity anomaly shows an NE-striking high-low-high pattern from northwest to southeast, strongly related to the main faults. According to the results of MSA, the correlation between gravity anomaly and faults is particularly significant from 4 to 12 km depth. The residual gravity with small amplitude in each layer indicates weak tectonic activity in the crust. In the upper layers, positive anomalies along most of faults reveal the upwelling of high-density materials during the past tectonic movements. The multilayer residual anomalies also yield important information about the faults, such as the vertical extension and the dip direction. The maximum depth of the faults is about 20 km. In general, NE-striking faults extend deeper than NW-striking faults and have a larger dip angle.
Chuang Xu; Haihong Wang; Zhicai Luo; Hualiang Liu; Xiangdong Liu. Insight into Urban Faults by Wavelet Multi-Scale Analysis and Modeling of Gravity Data in Shenzhen, China. Journal of Earth Science 2017, 29, 1340 -1348.
AMA StyleChuang Xu, Haihong Wang, Zhicai Luo, Hualiang Liu, Xiangdong Liu. Insight into Urban Faults by Wavelet Multi-Scale Analysis and Modeling of Gravity Data in Shenzhen, China. Journal of Earth Science. 2017; 29 (6):1340-1348.
Chicago/Turabian StyleChuang Xu; Haihong Wang; Zhicai Luo; Hualiang Liu; Xiangdong Liu. 2017. "Insight into Urban Faults by Wavelet Multi-Scale Analysis and Modeling of Gravity Data in Shenzhen, China." Journal of Earth Science 29, no. 6: 1340-1348.
Using continuous 1-Hz sampling time-series recorded by a SG (superconducting gravimeter) at Hsinchu station, Taiwan of China, we investigate the anomalous gravity signals prior to 71 large earthquakes with moment magnitude larger than 7.0 (Mw7.0) occurred between 1 Jan 2008 and 31 Dec 2011. We firstly evaluate the noise level of the SG records at Hsinchu (HS) station in microseismic bands from 0.05 Hz to 0.1 Hz by computing the PSD (power spectral density) of seismically quiet days selected based on the RMS of records. Based on the analysis of the noise level and the spectral features of the seismically quiet SG records at HS station, we detect AGSs (anomalous gravity signals) prior to large earthquakes. We apply HHT (Hilbert-Huang transformation) to establish the TFEP (time-frequency-energy paradigms) and MS (marginal spectra) of the SG data before the large earthquakes, and the characteristics of TFEP and MS of the SGs data during the typhoon event are also analyzed. By comparing the spectral characteristics of the SGs data during seismically quiet period, three types of AGSs are found; and the occurrence rate of AGSs before 71 earthquakes is given in terms of the cases with different epicenter distance and different focal depth. The statistical results show that 56.3% of all the examined large earthquakes were preceded by AGSs; and if we constrain the epicenter distance to be smaller than 3500 km and focal depth less than 300 km, 75.3% of the examined large earthquakes can be associated with the AGSs. Especially, we note that for all the large earthquakes occurred in the Eurasian plate in recent four years, the precursory AGSs can always be found in the SG data recorded at HS station. Our investigations suggest that the AGSs prior to large earthquakes may be related to focal depth, epicentre distance and location
Dijin Wang; Cheinway Hwang; Wenbin Shen. Investigations of anomalous gravity signals prior to 71 large earthquakes based on a 4-years long superconducting gravimeter records. Geodesy and Geodynamics 2017, 8, 319 -327.
AMA StyleDijin Wang, Cheinway Hwang, Wenbin Shen. Investigations of anomalous gravity signals prior to 71 large earthquakes based on a 4-years long superconducting gravimeter records. Geodesy and Geodynamics. 2017; 8 (5):319-327.
Chicago/Turabian StyleDijin Wang; Cheinway Hwang; Wenbin Shen. 2017. "Investigations of anomalous gravity signals prior to 71 large earthquakes based on a 4-years long superconducting gravimeter records." Geodesy and Geodynamics 8, no. 5: 319-327.
Determining the Moho topography of the Tibetan Plateau is crucial to understand the tectonic development. Over the past decades, seismic explorations have obtained profound results about the Moho topography, except in regions where seismic station coverage is poor, especially in the central and western Tibetan Plateau. In comparison, gravity data have the advantage of global homogeneous coverage, which can thus be used to determine the Moho structure beneath the entire Tibetan Plateau. In this paper, a novel approach, the multi-scale gravity analysis method, is developed to extract the gravity signals originated from the Moho undulations and to determine the Moho topography beneath the whole Tibetan Plateau. The inverted Moho topography for the Tibetan Plateau is consistent with that derived from the previous works. In addition, a rich geophysical structure and tectonic development can be revealed from the inverted Moho topography: (1) The Moho depth in the west is deeper than that in the east, and the deepest Moho, which is approximately 77 km, is located beneath the western Qiangtang Block; (2) There is an obvious Moho offset of approximately 5 km beneath the Yarlung-Zangbo Suture at the juncture between the Himalayan and Lhasa Blocks; (3) The Moho fold and low-density channel flow, the directions of which are in agreement with the results of surface movement velocities estimated from Global Positioning System, can be observed from the Moho topography.
Chuang Xu; Ziwei Liu; Zhicai Luo; Yihao Wu; Haihong Wang. Moho topography of the Tibetan Plateau using multi-scale gravity analysis and its tectonic implications. Journal of Asian Earth Sciences 2017, 138, 378 -386.
AMA StyleChuang Xu, Ziwei Liu, Zhicai Luo, Yihao Wu, Haihong Wang. Moho topography of the Tibetan Plateau using multi-scale gravity analysis and its tectonic implications. Journal of Asian Earth Sciences. 2017; 138 ():378-386.
Chicago/Turabian StyleChuang Xu; Ziwei Liu; Zhicai Luo; Yihao Wu; Haihong Wang. 2017. "Moho topography of the Tibetan Plateau using multi-scale gravity analysis and its tectonic implications." Journal of Asian Earth Sciences 138, no. : 378-386.